With environmental problems in the background, a need for improving fuel economy has increased. For parts for mechanical structures used for motor vehicles, industrial machines, and the like, the increase in strength of part has been desired in order to reduce the size thereof.
In recent years, the regulation of exhaust gas for motor vehicles tends to be increasingly stricter. For a diesel engine fuel injection system, the combustion efficiency of engine can be enhanced by increasing the injection pressure of fuel. Accordingly, the injection pressure of fuel injected into a diesel engine has been raised. A common rail is a hollow shaped part that is used for the diesel engine fuel injection system and temporarily stores the pressurized fuel before the fuel is injected into the engine.
The interior of common rail is repeatedly subjected to a high internal pressure. Therefore, a steel material used for the common rail is required to have a high fatigue strength against the internal pressure, to have a high fracture toughness to prevent brittle fracture even if a fatigue crack is generated by the repeatedly applied internal pressure, to have high machinability to facilitate the formation of a plurality of intersecting holes formed in the part, and so on. With the increase in injection pressure of fuel injection system, further enhancement of performance has been desired on the steel material used for the common rail as well.
On the other hand, from the viewpoint of production cost of parts, it is desirable to use, for the common rail, a non-thermally refined steel material in which a steel bar produced by hot rolling (hereinafter, a steel bar as is hot-rolled, which steel bar is produced by hot rolling is referred to as a “rolled steel bar”) is formed by hot forging (hereinafter, a rolled steel bar as is formed by hot forging is referred to as a “hot-forged section material”), and a desired strength can be obtained without performing heat treatment of quenching and tempering, that is, “thermal refining treatment”.
Thus, as a steel material used for a common rail, it is desired to apply the rolled steel bar that can be formed into a part shape by cutting work before use, without thermal refining treatment after the hot-forged section material has been produced by hot forging.
So far, various techniques for improving the fatigue strength and the like of a part used for the fuel injection system have been proposed.
Patent Document 1 discloses a free cutting steel that contains Bi and S as inclusion forming elements, and is provided with both of high fatigue strength and excellent machinability, and a fuel injection system part using the free cutting steel.
Patent Document 2 discloses a steel for common rail excellent in fatigue properties, in which REM is contained, and the dispersion mode of sulfide-based inclusions, nitride-based inclusions, and oxide-based inclusions is controlled, and a common rail.
Patent Document 3 discloses a steel-made high-strength fabricated product excellent in shock resistance and balance of strength-ductility, in which a steel material containing a proper amount of one or more elements selected from a group consisting of Nb, Ti and V and a proper amount of Al is used, and the metal micro-structure of the steel material is made to consist of ferrite, retained austenite, and bainite and/or martensite by controlling the cooling after hot forging.
Patent Document 4 discloses a steel excellent in fatigue properties in which the length-to-width ratio of Mn sulfide-based inclusion is made a certain value or lower, and a steel part produced from the steel.
Patent Document 5 discloses a ferrite/pearlite type non-thermally refined steel for hot forging, in which the contents of C, S and V are especially controlled, and the fatigue strength and the cutting workability using a cemented carbide drill are excellent, and a common rail using the non-thermally refined steel.